The present invention relates to a method in bleaching of pulp, and more specifically the invention concerns a method in which the pulp is subjected to a peroxide bleaching step, whereupon the pulp is washed and the washing liquid from the washing is separated from the pulp in the form of a filtrate.
Pulp, which here relates to cellulose-containing pulp for making paper, is normally subjected to bleaching in order to dissolve lignin and/or increase the brightness of the pulp. The bleaching of pulp is usually carried out in a plurality of steps, such as 4-5 steps when bleaching sulphate pulp or 1-2 steps when bleaching mechanical pulp, and in peroxide bleaching the bleaching is carried out in at least one of these steps with hydrogen peroxide under alkaline conditions. An increasing amount of papermaking pulp is currently bleached by means of hydrogen peroxide. The hydrogen peroxide bleaching can be carried out at atmospheric pressure and a temperature of about 70-100xc2x0 C. or be pressurised, for instance to a pressure of 5 bar with oxygen, and be carried out at a temperature above 100xc2x0 C., such as 100-120xc2x0 C. A conventional peroxide bleaching method for sulphate pulp can comprise, for instance, the following steps: an oxygen step, in which the pulp is treated with oxygen, followed by washing of the pulp before treating it with a complexing agent, such as ethylene diamine tetraacetic acid (EDTA) to complex interfering metal ions, such as iron, manganese, copper, zinc and chromium. The treatment with a complexing agent can be carried out in one or more steps. Subsequently, the pulp is washed and then fed to a peroxide bleaching step, where the pulp is bleached with hydrogen peroxide. After the peroxide bleaching step, the pulp is washed and after separation of the filtrate, this can at least partly be recirculated to the peroxide bleaching step as washing liquid. The various washing steps that occur in connection with the bleaching are generally carried out with a washing liquid (water) which is fed countercurrently to the process flow, i.e. clean washing water is added at the end of the bleaching and is supplied in the direction of the beginning of the bleaching, the washing liquid being successively used to wash more and more contaminated (unbleached) pulp. In this manner, the washing liquid can be utilised optimally.
The efficiency of the hydrogen peroxide in the peroxide bleaching is impaired by interfering substances, such as oxygen-demanding organic material (below referred to as COD material, i.e. chemically oxygen-demanding material) and metal ions, such as manganese, iron, chromium, copper, nickel and zinc (cf. Gellerstedt, G., Pettersson, L., Chemical Aspects of Hydrogen Peroxide Bleaching, Part 2. The bleaching of kraft pulps. Journal of Wood Chemistry and Technology 2 (1982) :3, 231). This means that the hydrogen peroxide must be supplied to the bleaching step in excess to be able to achieve the intended bleaching effect. The negative effect of interfering substances in the peroxide bleaching step increases by recirculation of the filtrate from the washing of the pulp after the peroxide bleaching step by the filtrate containing interfering substances from the peroxide bleaching step, and in continued recirculation of the filtrate an increasing amount of interfering substances is collected in the filtrate. Thus, hydrogen peroxide must normally be added to the bleaching step in a great excess, and it is as a rule estimated that about 40% of the charged hydrogen peroxide is consumed for bleaching while about 40% decomposes owing to the presence of interfering substances (cf. J. Hxc3x6xc3x6k and U. Ekholm, xe2x80x9cMicrocalorimetryxe2x80x94a New Tool to Study Hydrogen Peroxide Reactionsxe2x80x9d, 9th International Symposium on Wood and Pulping Chemistry: 1997). This means that the filtrate which is obtained after washing and dewatering of the pulp after the peroxide bleaching step contains only about 20% of remaining hydrogen peroxide together with oxygen-demanding organic material.
In the present invention, it has now surprisingly been found that the problem of undesirable decomposition of hydrogen peroxide in the peroxide bleaching step can be significantly reduced by treating the filtrate from the dewatering of the pulp after the peroxide bleaching step with an aluminium compound for precipitation of interfering substances, such as COD material, before the filtrate is recirculated to the peroxide bleaching step.
More specifically, the present invention provides a method in bleaching of pulp, in which the pulp is subjected to a peroxide bleaching step, whereupon the pulp is washed and the washing liquid from the washing is separated from the pulp in the form of a filtrate, characterised in that the filtrate is treated with an aluminium compound for precipitating interfering substances, that the precipitated material is removed from the filtrate, that the aluminium content of the filtrate after precipitation of interfering substances is adjusted to at most 20 ppm, and that the filtrate is then recirculated to the peroxide bleaching step.
Further features and advantages of the invention are evident from the following description and the appended claims.
By the expression xe2x80x9cpulpxe2x80x9d used in the context is meant a cellulose-containing pulp for making paper, said pulp comprising mechanical pulp as well as semichemical and chemical pulp, such as sulphate pulp and sulphite pulp.
By the expression xe2x80x9cperoxide bleachingxe2x80x9d which is used in the context is meant bleaching with an alkali peroxide or, preferably, hydrogen peroxide as the bleaching chemical.
The aluminium compound which is used for treatment of the filtrate is not particularly critical and can be selected among a large number of different aluminium compounds, such as those known as flocculating or precipitating agents in connection with the purification of drinking water and waste water. As specific examples of usable aluminium compounds mention can be made of aluminium chloride, aluminium nitrate, aluminium sulphate, as well as polyaluminium compounds, such as polyaluminium chloride, polyaluminium nitrate and polyaluminium sulphate. Of these, however, aluminium chloride is less preferred since it may cause corrosion. Particularly preferred aluminium compounds are aluminium sulphate, polyaluminium nitrate and polyaluminium sulphate.
The amount of aluminium compound which is added to the filtrate should at least be sufficient to reduce and, if possible, essentially eliminate interfering substances, such as oxygen-demanding organic material (COD material) as well as metals, by precipitation from the filtrate. In general terms, this means an addition of about 1-6000 mg Al/kg of pulp, preferably 1000-4000 mg Al/kg of pulp, more preferred 1500-3500 mg Al/kg of pulp, and most preferred about 2500 mg Al/kg of pulp. This corresponds generally to about ca 1-600 mg (ppm) Al/l of filtrate, preferably 100-400 mg Al/l of filtrate, more preferred 150-350 mg Al/l of filtrate, and most preferred about 250 mg Al/l of filtrate.
The aluminium compound used for precipitation should, especially if it contains aluminium in loosely bound/dissociable form, not be added in such an excess that considerable amounts of non-reacted aluminium compound, after removal of precipitated material from the filtrate, accompany the filtrate to the peroxide bleaching step. It has, in fact, been established in the invention that the aluminium compound used for the precipitation can have a negative effect on the peroxide bleaching so that the consumption of peroxide increases. The aluminium compound is added in such an amount that the filtrate after removal of precipitated material contains at most about 20 ppm Al, preferably at most about 10 ppm Al, more preferred at most about 7 ppm Al.
To prevent an undesirable excess of the aluminium compound from accompanying the filtrate to the peroxide bleaching step, an undesirable excess of the aluminium compound can, after the precipitation of interfering substances, be precipitated as a sparingly soluble aluminium compound.
The temperature in the treatment of the filtrate with the aluminium compound is not particularly critical, and the treatment is normally carried out at the temperature that the filtrate has after washing of the pulp from the peroxide bleaching step. This temperature is usually in the range 60-100xc2x0 C., preferably 70-90xc2x0 C.
The treatment of the filtrate with the aluminium compound can be carried out within a wide pH range. As a rule, however, the pH should be in the range 4-11, preferably 5-7 with regard to good precipitation and flocculation of interfering substances, such as organic oxygen-demanding material (COD) and metals, and to the fact that the pH in the peroxide bleaching preferably is in the range 8-11, more preferred 9-11. If necessary, the pH of the filtrate can be adjusted, the adjustment suitably being effected by using an alkali, such as sodium hydroxide, or an acid, such as sulphuric acid.
According to the invention, the treatment of the filtrate with the aluminium compound is suitably carried out in such manner that the desired amount of aluminium compound is added to the filtrate under agitation so that the aluminium compound is distributed uniformly in the filtrate for precipitation and flocculation of interfering substances. For the precipitation to be optimal, the aluminium compound should be given sufficient time to act before the formed precipitate is separated from the filtrate, for instance by filtration. Suitably a time from about 1 s to about 30 min, preferably about 1-5 min, should be allowed to pass from the addition of the aluminium compound to the separation of the precipitate.
With a view to facilitating the flocculation, a flocculating aid in the form of a polymer can be added. Such flocculating aids are well known to those skilled in the art and comprise, for instance, polyacrylamide.
After treatment of the filtrate with an aluminium compound and separation of the formed precipitate, the filtrate is recirculated to the peroxide bleaching step. This recirculation or reuse of the filtrate in the peroxide bleaching step is carried out by the filtrate being supplied countercurrently to the process flow and being used as the washing liquid for washing of pulp in a washing step preceding the peroxide bleaching step.
Owing to the separation of the formed precipitate, the filtrate contains fewer substances that have a detrimental effect on the peroxide bleaching, and the recirculation of the filtrate to the peroxide bleaching step can therefore be increased compared with the recirculation of filtrate in a conventional peroxide bleaching method.
By the filtrate being purified, the hydrogen peroxide content in the peroxide bleaching should be affected positively, i.e. the filtrate in the washing of the pulp after the peroxide bleaching step should contain a higher remaining peroxide content than according to conventional technique. This condition has also been established, but in the invention also an unexpected effect which has not been explained until now has been established, which means that the remaining amount of peroxide in the filtrate after the peroxide bleaching step is further increased. This is supposed to be due to the fact that in the filtrate treated with the aluminium compound, small amounts of another, aluminium-containing compound form, which is capable of binding metal ions, such as manganese, so that they cannot have any peroxide-decomposing effect in the peroxide bleaching step. By small amounts is here meant amounts in the order of at most 20 ppm Al, such as 0.01-20 ppm Al, preferably 0.01-10 ppm Al, more preferred 0.01-7 ppm Al. During the bleaching step, the aluminium in the aluminium-containing compound appears to work to prevent unwanted decomposition of hydrogen peroxide. This, in turn, is assumed to be due to the fact that the aluminium-containing compound is of a type which together with aluminium contains an alkaline earth metal, such as magnesium, calcium or barium, preferably magnesium, which is normally present in the pulp. In compounds containing magnesium together with aluminium, it is preferred for the molar ratio Mg:Al to be from 3:1 to 6:1. An example of such a compound is hydro-talcite having the general formula
[Mg1xe2x88x92xAlx(OH)2]x+[(CO3)x/2.nH2O]xxe2x88x92
wherein
0.10 less than xc3x97 less than 0.34
n=1xe2x88x923x/2.
The structure of hydrotalcite is composed of infinite layers, on the one hand Mg(OH)2 layers where a certain part of Mg2+ has been replaced with Al3+ and, on the other hand, intermediate layers with CO32xe2x88x92 and H2O. The amount of carbonate ions corresponds to half the amount of aluminium (in moles) since they constitute a charge equalisation when 2-valent magnesium ions are substituted with 3-valent aluminium ions. The carbonate ions can be replaced with other anions of the type SO42xe2x88x92.
In the invention, a surprising synergistic action has been found, which means that the filtrate from the washing of the pulp after the peroxide bleaching can contain 50% of remaining hydrogen peroxide or more. Since about 40% of the charged hydrogen peroxide is still necessary for the actual bleaching, this means that the undesirable decomposition of hydrogen peroxide according to the invention is reduced drastically from about 40% to about 10% only.